1. Field of the Invention
This invention relates to an electrochromic display having a high response speed, an excellent contrast, a high resolving power and a long service life. More specifically, it relates to an improved electrochromic display containing a bipolar ion-exchange membrane as an electrolyte layer.
2. Description of the Prior Art
The electrochromic display, to be sometimes abbreviated "ECD" hereinafter, is a display which makes use of a reversible change in color that takes place in an inorganic or organic substance upon the application of voltage. It is easier to view than liquid crystal displays because it has no dependence upon visual field angles. It is also characterized, for example, by having a memory function and being able to be driven at low voltages. From the practical standpoint, however, its service life, response speed and contrast have not proved to be entirely satisfactory, and it has been desired to develop ECD which is satisfactory in these three factors.
It is known that ECD is generally composed of a transparent electroconductive membrane, an electrochromic membrane, an electrolyte layer and a counter electrode in a laminated structure. The life, response speed and contrast of ECD depend greatly not only upon the kind of a substance constituting the electrochromic membrane, but also upon the kind of a substance constituting the electrolyte layer.
Various electrolytes have been proposed heretofore for use in the electrolyte layer. Typical examples include liquid electrolytes such as an aqueous solution of sulfuric acid and a propylene carbonate solution of lithium perchlorate (LiClO.sub.4), solid electrolytes such as antimonic acid, tantalum oxide and magnesium fluoride, and ion-exchange membranes.
ECD comprising an electrolyte layer containing a liquid electrolyte generally has good response speeds and contrasts, but is inferior in service life because of the dissolution of the electrochromic membrane in the liquid electrolyte and the problem of the counter electrode. ECD comprising a solid electrolyte layer has a long service life, but is inferior in response speed and contrast. When an ion-exchange membrane is used as the electrolyte layer, ECD having a high response speed and a long life may be obtained depending upon the type of the ion-exchange membrane.
The formidable problem associated with displaying by these electrochromic displays is that since the correspondence of the degree of coloration to an applied voltage is linear (namely, the voltage and the current are in a linear relationship), these electrochromic displays have a low resolving power in displaying fine details and cannot give clear displayed images. For example, in displaying fine details such as characters and figures in ECD, there is employed a method in which the display is divided into a plurality of segments, and voltage is applied to a combination of the required segments. With this method, however, color blurring tends to occur in coloring and discoloring characters, figures, etc. and leads to the inability to provide clear display images. In the case of using the ion-exchange membrane in the electrolyte layer, coloration and discoloration cannot be maintained clear during long-term use, or the contrast may become insufficient.